The present invention is related to an isochronous cyclotron that can be a compact isochronous cyclotron as well as a separate sector cyclotron.
The present invention applies both to super-conducting and non-super-conducting cyclotrons.
The present invention is also related to a new method to extract charged particles from an isochronous sector-focused cyclotron.
A cyclotron is a circular particle accelerator which is used to accelerate positive or negative ions up to energies of a few MeV or more. Cyclotrons can be used for medical applications (production of radioisotopes or for proton therapy) but also for industrial applications, as injector into another accelerator, or for fundamental research.
A cyclotron consists of several sub-systems of which the most important are mainly the magnetic circuit; the RF acceleration system, the vacuum system, the injection system and the extraction system.
The most important is the magnetic circuit by which a magnetic field is created. This magnetic field guides the accelerated particles from the centre of the machine towards the outer radius of the machine in such a way that the orbits of the particles describe a spiral. In the earliest cyclotrons the magnetic field was created in a vertical gap between two cylindrically shaped poles by two solenoid coils wound around these poles. In more recent isochronous cyclotrons these poles no longer consist of one solid cylinder, but are divided into sectors such that the circulating beam alternately experiences a high magnetic field created in a hill sector where the gap between the poles is small, followed by a lower magnetic field in a valley sector where the gap between the poles is large. This azimuthal magnetic field variation, when properly designed, provides radial as well as vertical focusing and at the same time allows the particle revolution frequency to be constant throughout the machine.
Two types of isochronous cyclotrons exist: the first type is the compact cyclotron where the magnetic field is created by one set of circular coils wound around the total pole; the second type is the separate sector cyclotron where each sector is provided with its own set of coils.
Document EP-A-0222786 describes a compact sector-focused isochronous cyclotron, called xe2x80x9cdeep-valley cyclotronxe2x80x9d, which has a very low electrical power consumption in the coils. This is achieved by a specific magnetic structure having a strongly reduced pole gap in the hill sectors and a very large pole gap in the valley sectors, combined with one circular shaped return yoke placed around the coils which serves to close the magnetic circuit.
Document WO93/10651 describes a compact sector-focused isochronous cyclotron having the special feature of an elliptically or quasi-elliptically shaped pole gap in the hill sectors which tends to close towards the outer radius of the hill sector and which allows to accelerate the particles very close to the outer radius of the hill sector without losing the focusing action and the isochronism of the magnetic field. This will facilitate the extraction of the beam as is pointed out later.
The second main sub-system of a cyclotron is the RF accelerating system which consists of resonating radio-frequency cavities which are terminated by the accelerating electrodes, usually called the xe2x80x9cdeesxe2x80x9d. The RF system creates an alternating voltage of several tenths of kilovolts on the dees at a frequency which is equal to the revolution frequency of the particle or a higher harmonic thereof. This alternating voltage is used to accelerate the particle when it is spiralling outwards to the edge of the pole. Another main advantage of the deep-valley cyclotron is that the RF-cavities and dees can be placed in the valleys, allowing for a very compact design of the cyclotron.
The third main sub-system of a cyclotron is the vacuum system. The purpose of the vacuum system is to evacuate the air in the gap where the particles are moving in order to avoid too much scattering of the accelerating particles by the rest-gas in the vacuum tank and also to prevent electrical sparks and discharges created by the RF system.
The fourth sub-system is the injection system which consists basically of an ion source in which the charged particles are created before starting the accelerating process. The ion source can be mounted internally in the centre of the cyclotron or it can be installed outside of the machine. In the latter case the injection system also includes the means to guide the particles from the ion source to the centre of the cyclotron where they start the acceleration process.
When the particles have completed the acceleration and have reached the outer radius of the pole sectors they can be extracted from the machine, or they can be used in the machine itself. In the latter case an isotope production target is mounted in the vacuum chamber. The main disadvantage of this is however, that the particles partly scatter away from the target and then become lost in an uncontrolled manner all over the vacuum tank. This may cause a strong radio-activation of the machine.
In many applications it is wished to bring the beam outside of the machine and guide it to a target where it can be used. In this case an extraction system is installed near the outer radius in the machine. The beam extraction is considered as one of the most difficult processes in generating a cyclotron beam. It basically consists in bringing the beam in a controlled manner from the acceleration region to an outer radius where the magnetic field is low enough so that the beam can freely exit the machine.
For extracting positively charged particles the common method is to use an electrostatic deflector which produces on outward electric field which pulls the particles out of the confining influence of the magnetic field. To achieve this action, a very thin electrode called septum is placed between the last internal orbit in the machine and the orbit that will be extracted. However, this septum always intercepts a certain fraction of the beam and therefore this extraction method has two main drawbacks. The first one is that the extraction efficiency is limited, thereby limiting the maximum beam intensity that can be extracted due to thermal heating of the septum by the intercepted beam. The second is that interception of particles by the septum contributes strongly to the radio-activation of the cyclotron.
Another well known extraction method concerns negatively charged particles. Here the extraction is obtained by passing the beam through a thin foil wherein the negative ions are stripped from their electrons and are converted into positive ions. This technique allows for an extraction efficiency close to 100% and furthermore an extraction system which is considerably simpler then the previous one. However, also here there is a main disadvantage caused by the fact that the negative ions are not very stable and therefore easily get lost by collisions with the rest gas in the vacuum tank or by too large magnetic forces acting on the ion. This beam loss again causes unwanted radio-activation of the cyclotron. Furthermore, cyclotrons accelerating positive ions allow to produce higher beam intensities with a higher reliability of the accelerator and at the same time allow a strong reduction in size and weight of the machine.
Also known from the publication xe2x80x9cThe Review of Scientific Instruments, 27 (1956), No. 7xe2x80x9d and from the publication xe2x80x9cNuclear Instruments and Methods 18, 19 (1962), pp. 41-45e by J. Reginald Richardson, is a claim of a method where the beam could be extracted from the cyclotron without the use of an extraction system. The conditions needed for this auto-extraction are certain resonance conditions of the particle orbits in the magnetic field. However, this method will be difficult to realise and also would give such a bad extracted optical beam quality that in practice it will never be applied.
Also known is the document U.S. Pat. No. 3024379 which reports on a cyclotron system in which the magnetic field is essentially independent on the azimuthal angle. This means that this is a non-isochronous cyclotron. It should be noted that the cyclotron described here includes means for extraction of the beam that consists of xe2x80x9cregeneratorsxe2x80x9d and xe2x80x9ccompressorsxe2x80x9d which allow, by perturbing the magnetic field, an extraction of the beam.
 Document EP-0853867 describes a method for extraction from a cyclotron in which the ratio between the pole gap in the hill sector near the maximum radius and the radial gain per turn of the particles at the same radius is lower than 20 and in which the pole gap in the hill sector has an elliptical or quasi-elliptical shape with a tendency to close at the maximum radius of the hill sector and in which at least one of the hill sectors has a geometrical shape or a magnetic field which is essentially asymmetric as compared to the other hill sectors. The present invention relies among others on this narrow quasi-elliptical pole gap and the asymmetry of at least one sector and at the same time outlines the kind of asymmetries that can be applied to obtain the auto-extraction of the beam.
The aim of the present invention is to propose a new method for extraction of charged particles from a cyclotron without using a stripping mechanism or an electrostatic deflector as it has been described above.
An additional aim is to obtain in this way an isochronous cyclotron who is more simple in concept and also more economical than those which are presently available.
Another additional aim is to increase the extraction efficiency and the maximum extracted beam intensity especially for positively charged particles.
The present invention is related to a superconducting or non-superconducting isochronous sector-focused cyclotron, comprising an electromagnet with an upper pole and a lower pole that constitutes the magnetic circuit, the poles being made of at least three pairs of sectors called xe2x80x9chillsxe2x80x9d where the vertical gap between said sectors is small, these hill-sectors being separated by sector-formed spaces called xe2x80x9cvalleysxe2x80x9d where the vertical gap is large, said cyclotron being energised by at least one pair of main coils, characterised in that at least one pair of upper and lower hills is significantly longer than the remaining pair(s) of hill sectors in order to have at least one pair of extended hill sectors and at least one pair of non-extended hill sectors and in that a groove or a xe2x80x9cplateauxe2x80x9d which follows the shape of the extracted orbit is present in said pair of extended hill sectors in order to produce a dip in the magnetic field.
According to one preferred embodiment, the radial width of the groove is limited to a few centimetres, preferably of the order of 2 cm, such that it is completely located on the extended hill sector.
According to an alternative embodiment, the outer border of the groove may also be moved beyond the radial extremity of the extended hill sector, in which case a kind of xe2x80x9cplateauxe2x80x9d is formed which is however still characterised by the stepwise increase of the vertical hill gap and the related sudden decrease of the magnetic field near the inner border of the xe2x80x9cplateauxe2x80x9d.
Preferably, the vertical gap in the nonextended hill sectors as well as the vertical gap in the extended hill sectors has essentially an elliptical profile which tends to close towards the median plane at the radial extremity of the hill sectors.
According to one preferred embodiment, at least one set of harmonic coils is placed in the vertical hill gap, said coils having essentially the shape of the local orbit at that place. Said coils serving to add a first harmonic field component to the existing magnetic field and to increase the turn separation at the entrance of the groove.
According to another preferred embodiment, the vertical hill gap profiles onto azymuthally opposite hill sectors is deformed such that one profile shows a profound bump on the last turn of the orbit and the other profile shows a profound dip on the last turn of the orbit. Said deformation serves to add a first harmonic field component to the existing magnetic field and to increase the turn separation at the entrance of the groove.
According to a third preferred embodiment, an arrangement of permanent magnets is placed in two opposite valleys such that in one valley a sharp magnetic field bump is created on the last turn of the orbit and in the opposite valley a magnetic field dip is created on the last turn of the orbit. Said arrangement serves to add a first harmonic field component to the existing magnetic field and to increase the turn separation at the entrance of the groove.
Preferably, a gradient corrector will be present at the exit of the groove. Such gradient corrector comprises unshielded permanent magnets and shows a completely open vertical gap as well as small compensating permanent magnets in order to minimise the perturbing magnetic field at the internal orbit.
Advantageously, a lost beam stop is provided behind the exit of the gradient corrector at an azimuth where there is a significant turn separation between the extracted beam and the last turn of the orbit. Said beam stop is placed such that it intercepts the bad parts of the internal beam as well as the extracted beam.
Preferably, in the return yoke, a pair of horizontally and vertically focusing quadrupoles is placed after the vacuum exit port which are made of unshielded permanent magnets.
The present invention is also related to a method for the extraction of a charged particle beam from a isochronous sector-focused cyclotron as described hereabove, wherein a sharp dip in the magnetic field on the last turn of the orbit will be used in order to extract the beam of particles without the help of an electrostatic deflector or a stripper mechanism.